1. Field of the Invention
This invention relates to current sources and converters for electronic devices and particularly to identifying current sources and converters.
2. Description of the Related Art
Electronic devices, and particularly portable electronic devices such as portable computers, cellular telephones, and personal digital assistants (PDAs), typically use current converters to convert electrical current from a current source into a current type (e.g., alternating current (AC) and direct current (DC)), voltage, and/or current that is appropriate for the electronic device's operation. Converters are also used to charge on-board rechargeable batteries. For example, an AC to DC current converter ("AC-DC adapter," "AC adapter," or simply "adapter") typically plugs into a wall outlet and converts the 110-240 volts, 50-60 Hz AC current into a lower voltage DC current for use by an electronic device. Other current converters convert DC currents of one voltage, to a DC current of another voltage. Examples of this type of converter include automobile and airplane adapters that plug into a DC current source in the vehicle, and supply a different DC current (e.g., different voltage) to an electronic device. Still another example of a current converter is a DC to AC converter, or "inverter." Such devices are particularly useful when a user is presented with a DC source, such as the standard cigarette lighter connector in an automobile, but the electronic device to be used already includes an internal AC to DC adapter, such that the device requires AC current as input. Finally, AC to AC converters are also used, for example, to convert the standard AC current of one country to that of another country.
Current converters are commonly provided as a separate module with a plug or cord for connecting the converter to the current source (e.g., automobile cigarette lighter, airline power outlet, or AC wall outlet) and another cord for connecting the converter to the electronic device through a connector. Given the variety of electronic devices that use converters, and the various output polarizations, voltage ratings, and current ratings of those converters, an electronic device user is likely to have several, if not many, different converters for different electronic devices. Consequently, matching the correct converter to the intended device can be difficult because of similarity in appearance among converters and similarity among the connectors associated with the converters. Compounding this problem is the fact that converters intended for different applications can be manufactured by the same company and look the same, yet have dissimilar electrical characteristics.
Using the wrong converter with an electronic device can lead to improper performance of, and damage to, the electronic device. Moreover, using the wrong converter can present a safety hazard (e.g., electrical shock or fire) which can be of even greater concern when the electronic device is being used under extreme temperature conditions, such as in a parked automobile, or when the device is being used in a sensitive environment, such as on-board an aircraft while the aircraft is in flight. For example, use of an incorrect converter to apply current to a portable computer which, in turn, uses the current to charge a rechargeable battery can cause the battery to explode and the portable computer to catch fire. Under normal circumstances, such an event is quite serious, but on board an aircraft in flight, the incident can be life-threatening. Of equal concern is the situation where the correct current converter for the system is used, but the system includes an incorrect or substandard battery. The potential dangers associated with improper charging of batteries on-board aircraft has prompted at least one industry group, the Portable Rechargeable Battery Association, to urge the Federal Aviation Administration (FAA) to prohibit recharging on flights. Additionally, there can be circumstances, such as the in-flight example above, where use of even a proper current converter should be more tightly controlled or even prevented.
Prior methods to prevent improper use of and/or mitigate the damage from improper use of a current converter generally fall into two categories: mechanical methods and electrical methods. The most common mechanical solution to the problem of improper converter use is to provide the converter and the electronic device using the converter with unique connector keying such that the wrong connector cannot be inserted into the electronic device. One drawback to this method of preventing improper use of a converter is that it prevents manufacturers from using standard connectors and converters which allow the manufacturer to avoid the high costs of tooling, testing, and providing a custom part.
Electrical solutions typically include circuitry for clamping the improper input voltage with a dissapative device such as a zener diode, a metal oxide varistor (MOV), or a junction diode. These devices will only work if the input power source has power limiting within the capability of the dissapative device, and thus there ability to protect a device is limited. Such specialized circuits or components add cost and complexity to the electronic device. Additionally, operating conditions within the specification of the dissapative device are not always met, so damage to the electronic device can still result from using an improper converter.
Accordingly, it is desirable to have a current converter that can be identified by an electronic device in order to prevent use of an improper converter with the device. Additionally, it is desirable to have such a current converter that can provide information to the electronic device so that the device can operate more safely and efficiently when a proper current converter is used with the device.